Raman Jayalakshmi, Mehrotra Sonali, Anand Ranjith P, Balaram Hemalatha
Molecular Biology and Genetics Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India.
Mol Biochem Parasitol. 2004 Nov;138(1):1-8. doi: 10.1016/j.molbiopara.2004.06.013.
Adenylosuccinate synthetase (AdSS) catalyses the Mg(2+) dependent formation of adenylosuccinate from IMP and aspartate, the reaction being driven by the hydrolysis of GTP to GDP. All characterized AdSS thus far exhibit a random kinetic mechanism. We present here kinetic evidence that unlike all other AdSS, Plasmodium falciparum AdSS (PfAdSS) has ordered substrate binding. Inhibition studies show that binding of GTP requires IMP binding while aspartate binds to the enzyme-IMP-GTP complex. A structural basis for this difference in mechanism is presented. Kinetically, PfAdSS is closer to the mouse acidic isozyme rather than to the mouse basic isozyme. The mouse acidic isozyme is thought to play a role in the purine nucleotide biosynthetic pathway. Regulation of PfAdSS in vivo can therefore, be expected to be similar to the mouse acidic isozyme, in agreement with the role of PfAdSS as the only pathway for the synthesis of adenine nucleotides in the parasite. However, PfAdSS differs from both the mammalian homologs in that fructose-1,6-bisphosphate, a potent inhibitor of the mammalian enzyme, is an activator of PfAdSS. The differences highlighted here are promising in terms of species-specific drug design, targeting this essential enzyme in the parasite.
腺苷酸琥珀酸合成酶（AdSS）催化由肌苷酸（IMP）和天冬氨酸依赖镁离子形成腺苷酸琥珀酸，该反应由三磷酸鸟苷（GTP）水解为二磷酸鸟苷（GDP）驱动。迄今为止，所有已鉴定的AdSS均表现出随机动力学机制。我们在此提供动力学证据，表明与所有其他AdSS不同，恶性疟原虫AdSS（PfAdSS）具有有序的底物结合。抑制研究表明，GTP的结合需要IMP的结合，而天冬氨酸则与酶-IMP-GTP复合物结合。本文还提出了这种机制差异的结构基础。从动力学角度来看，PfAdSS更接近小鼠酸性同工酶而非小鼠碱性同工酶。小鼠酸性同工酶被认为在嘌呤核苷酸生物合成途径中起作用。因此，可以预期PfAdSS在体内的调节与小鼠酸性同工酶相似，这与PfAdSS作为寄生虫中腺嘌呤核苷酸合成的唯一途径的作用一致。然而，PfAdSS与两种哺乳动物同源物的不同之处在于，果糖-1,6-二磷酸是哺乳动物酶的强效抑制剂，却是PfAdSS的激活剂。这里强调的差异在针对该寄生虫中这种必需酶的物种特异性药物设计方面很有前景。
